1. Field of the Invention
The present invention relates to a power factor compensating single-stage converter, and more particularly, to a single-stage converter which improves the power factor by connecting an input voltage to a predetermined location of the primary winding of a transformer through a diode and a feedback inductor.
2. Description of the Related Art
Generally, a rectifier receives an ac voltage and outputs a dc voltage. Also, a dc-dc converter converts a dc voltage to an ac voltage, raises or lowers the voltage, and rectifies the ac voltage. This converter uses a pulse-width modulation (PWM) method. The types of converter include a flyback converter, a forward converter, a half-bridge converter, and a full-bridge converter.
Since today""s electronics apparatuses have characteristics that change very sensitively to the ripple of a dc voltage, researches to reduce the ripple of an output dc voltage continue. In a prior art embodiment for reducing the ripple of the output dc voltage, a condenser input-type rectifying method is used when an ac power is converted into a dc power. The condenser input-type rectifying method uses a large condenser comprising a full-bridge rectifier, the input terminal of which comprises four diodes. However, if a large condenser is used, due to the pulse-type charge current, the power factor of the ac input terminal is degraded down to 0.5-0.6. Since the power factor is increasingly controlled by regulations, degradation in the power factor is not desirable.
FIG. 1A is a schematic diagram of the structure of an embodiment of the prior art converter, and FIG. 1B is a waveform for explaining the operation of the prior art converter.
Referring to FIG. 1A, since in a half-cycle, diodes D1 and D4 flows current, and in another half-cycle, D2 and D3 flows current, the output waveform is as shown in FIG. 1B.
Referring to FIG. 1B, since in interval [0, t1] a voltage being supplied is greater than the voltage Vd that is charged to the condenser Cd, a charging current flows. Meanwhile, in interval [t1, t2] where the output voltage decreases, Cd discharges, and the time constant of the interval where the voltage decreases is the multiplication of load R0 and the capacitance Cd of the condenser.
Thus, interval [0, t1] for obtaining a charging current is much shorter than the discharging interval [t1, t2]. Therefore, the power factor of the converter shown in FIG. 1A is degraded because of the charging current of the condenser used to reduce the ripple of the output voltage. Also, since an input current flows only when the input voltage Vi is greater than the voltage between both ends of the condenser, the input current flows like a pulse around the maximum value of the input voltage and therefore contains more harmonic components compared to a sine wave current. To solve this problem, a power factor improving unit is introduced to the input part.
In a discontinuous current mode boost converter which is generally used, the current of the power source naturally follows the sine wave shape of the voltage of the power source. However, only when the output voltage of the power factor improving unit is far greater than the maximum value of the input voltage, the power factor improves. If the output voltage of the power factor improving unit increases, the voltage provided to semiconductor devices also increases, and in order to stand this voltage stress, a semiconductor switch having large voltage rating should be used. Meanwhile, since a semiconductor switch having large voltage rating also has large resistance, on-loss of the switch increases and accordingly the efficiency of the entire system is degraded. Therefore, development of a converter which can improve the efficiency of the entire system by reducing on-loss of the switch and also improving the power factor of an input terminal is very needed.
To solve the above problems, it is an objective of the present invention to provide a power factor compensating converter which improves the power factor by connecting a rectified input voltage to a predetermined location of the primary winding of a transformer through a diode and a feedback inductor.
To accomplish the objective of the present invention, there is provided a power factor compensating single-stage converter comprising a power factor improving unit which is connected to a predetermined input power source; a bridge diode unit which is located next to the power factor improving unit and provides a current path; a voltage smoothing condenser which stores electric energy provided through the bridge diode unit; a transformer circuit unit which is connected to the bridge diode unit and the voltage smoothing condenser; and a main switch which is connected to each of the bridge diode unit, the voltage smoothing condenser, and the transformer circuit unit and controls provision of voltage to the transformer circuit unit, wherein the power factor improving unit comprises two diodes connected to the input power source through a filter inductor, a condenser connected to two diodes, and a feedback inductor, one end of which is connected to a common contact of the two diodes and the other end of which is connected to a predetermined part of the primary winding of the transformer through a predetermined diode.
It is preferable that the main switch and the transformer circuit unit are constructed so that the converter is used as a flyback converter, a forward converter, or a half-bridge converter.